Late Quaternary evolution of piedmont faults in the Darjeeling-Sikkim Himalaya, India

Abstract

The collision between the Indian and Eurasian plates led to the development of foreland-propagating fold-and-thrust belts. The Main Frontal Thrust (MFT) is considered the youngest. However, geologists have also identified faults younger than the MFT in the foreland of the western Himalaya, which they named Piedmont faults. In this study, a similar fault is proposed in the foreland of the Darjeeling-Sikkim Himalaya, known as the Baradighi fault. Nakata (1972) first reported the scarp associated with the Baradighi fault, but little research has been conducted to understand its evolution. Sediment dating was performed using Optically Stimulated Luminescence (OSL) and ¹⁴C Accelerator Mass Spectrometry (AMS) methods. Results show that the activity on the Baradighi fault began around 30 ka and is still active. The region has traditionally been known as the ‘Siwalik Gap’ due to the absence of the Siwaliks, but this work reports the presence of the Siwaliks in the area. Geomorphic indices were used to assess the relative tectonic activity. The Asymmetry Factor indicated that most river basins are tilted westward. Basin elongation ratios suggest that not only E-W faults influence drainage, but also some active N-S aligned lineaments affect the fluvial system in the region. However, the hypsometric curve (HC) and index (HI) suggest that the area is not tectonically active, as most rivers originate in the unconsolidated Quaternary sediments, which are easily eroded. Consequently, deformation features caused by active tectonics have either been eroded or buried beneath river-deposited sediments.